A method and an inverter of said type is disclosed by T. Shimitzu et. al. in a paper at the conference proceedings of the IEEE Power Electronic Specialist Conference (PESC) 2002, p 1483-1488.
If an AC output voltage, which has an output frequency, is supplied to a resistive load an AC current will flow trough the load which has an identical waveform and phase with the AC voltage. Therefore, if the waveform is sinusoidal, the output power will fluctuate with a sinusoidal waveform with twice said output frequency. The DC source is loaded by a fluctuating power though it is able, as a matter of course, to supply a constant power. As a result, if the DC source is of a type, which converts energy from its environment to electrical energy, such as a solar cell array, an important part of the available energy from the environment will not be used and an overall efficiency of a system comprising such energy source will be poor. To solve this it is well known to buffer energy at the input side of the converter to equalize the power by which the source is loaded. However, such solution requires the use of large electrolytic capacitors as buffer. Such capacitors are expensive, have a large volume and have a limited lifetime, which is short with respect to an expected lifetime of a solar cell array, of for example 25 years. To solve this problem said article discloses the use of a DC power smoothing circuit, which is connected to receive the DC input voltage to reduce a ripple of it caused by a fluctuating output power. The smoothing circuit contains a capacitor, which is charged to twice the voltage of the DC input source. As a consequence, said capacitor can be smaller than the commonly used electrolytic capacitors. Apart from the disadvantage of using electrolytic capacitors the problem of the pulsating input/output power difference is not resolved.
The prior art inverter disclosed by said paper comprises a commonly known flyback converter, which is based on a commonly known buck-boost converter. With a flyback converter energy is buffered by a magnetic field in a transformer (with a buck-boost converter in a single inductor instead of said transformer). Energy is supplied from the DC source to a primary winding of the transformer during energy input intervals and taken from a secondary winding of the transformer during energy output intervals, which alternate the energy input intervals. This may be carried out at a high frequency, e.g. 200 kHz. Pulse width modulation (PWM) is used to shape a DC output voltage to conform with halve a cycle of the AC output voltage having a low frequency, e.g. 50 Hz. With the prior art inverter the secondary winding is balanced, such that its outer terminals of it are connected to a corresponding diode in series with a switch and then to a first terminal of a load, and a center tap of the secondary winding being connected to a second terminal of the load. The switches at the output side of the DC converter are controlled to concatenate the DC output voltages with opposite polarities during succeeding halves of the AC output voltage cycle.
A disadvantage of the prior art inverter is that it still requires the use of electrolytic capacitors and that it uses a switched smoothing buffer.
Another disadvantage of the prior art inverter is that it requires a transformer with a balanced secondary winding, two diodes and two switches at its output side. Therefore the inverter is relatively expensive.
Still another disadvantage of the prior art inverter is that it requires to be used in triplicate for providing a 3-phase AC output voltage. Therefore it requires a substantial amount of hardware.
Still, additional hardware would be required to synchronize the three AC output voltages and to phase shift them by 120° with respect to each other. Said paper does not disclose the application of the single phase inverter for a multi-phase arrangement. Consequently it does not disclose the need for synchronization and phase shifting and proper hardware for that either.